1. Field of the Invention
The invention relates to techniques of heating and bending glass sheets by gravity, in particular for the production of laminated automobile glazings.
2. Discussion of the Background
To bend simultaneously glass plates intended to be assembled and then glued with a plastic film, thus constituting a laminated safety glass, currently thermal bending by gravity is used. The same process has also been used to shape unit glass sheets intended to be associated with one or more plastic sheets. Also, an identical process has been used to shape unit glass sheets intended to undergo a thermal tempering treatment.
During the process of bending by gravity, the plane glass sheet or sheets, cut to the desired shape, are placed flat on a carriage consisting of a material supporting high temperatures without deformation--for example, stainless steel. This carriage moves discontinuously in the successive cells of a tunnel furnace; it advances step by step from one cell to the next, staying in each cell for a definite period (i.e., the cycle period). Over the course of time, the temperature of the glass sheet progressively rises until it first reaches then exceeds the transformation temperature. The glass then is deformed by the action of its own weight and approaches a rigid metallic form whose profile it assumes. This form, called a skeleton, can optionally be jointed to facilitate the deformation of the glass or even comprise devices that participate actively in the deformation--as described, for example, in European patent application No. EP 87 40 1344.
The furnace in which the carriages supporting the glass sheets move comprises two types of cells where they stay successively, each cell having its own thermal regulation. They are, in order: preheating cells in which the glass is heated as rapidly as possible, then bending cells where the glass temperature should be finely adjusted in the various zones of the glass sheet to make it possible for it to attain the exact shape desired. During this bending phase, heating elements commonly called "candles" are brought near the glass at varying distances. Thus, by selecting different distances, the various zones of the glass sheet can be heated differently. This device makes it possible not only to obtain varied radii of curvature as in a windshield, for example, but also, by changing the distances between the candles and the glass, from one carriage to the next, to obtain successively bending in different shapes. The great flexibility of this process therefore makes it possible by simple means to produce different models successively at the same installation. But a necessary condition must absolutely be met: the overall temperature of the glass sheet when it enters the first bending zone must be suitable for the process sequence. If it is too low, the energy applied locally during the bending will be insufficient to obtain the desired radii of curvature. If it is too high, even while keeping the candles in their farthest position, it will be impossible to limit sagging of the glass.
It is thus seen that a delicate problem which exists is that of obtaining an adequate temperature for the glass sheet at the exit of the preheating zone. In the case where the glass sheets treated successively are identical, the means usually used to obtain the right temperature at the end of the preheating consists in using a furnace whose cells are equipped at least partially with walls of heavy refractory materials which thus have a high heat capacity and whose temperature stays, because of this, remarkably stable, since the time each carriage remains in a cell is always the same (i.e., is the cycle period). To the extent that the heat quantities to be provided to each mobile system are of the same order, the temperature attained at the end of the cycle, at the exit from the cell, will be the same.
However, when it is desired to produce different glazings in the same furnace, the heat requirements of each mobile system can also be different. Thus, for example, some laminated windshields can consists of a glass 2 mm in thickness associated with another 1.5 mm in thickness, while other windshields will consist of two identical sheets 2.5 mm in thickness. The surfaces of the various models themselves being very different, it is seen that, actually, the weight of the windshield to be produced can vary from 10 kg to 25 kg. If the production series of the same type of windshield is long, the furnace is stabilized and the thermal conditions adapted to each type of glazing can be selected. However, for cars of small series or when the request is very diversified, as in the replacement products market, it can be desired to alternate the production of different models. Likewise, for production reasons, it can be desired to modify instantaneously the production program by introducing or eliminating at the last moment a given windshield in the programmed series. Then it would be necessary to be able to adapt instantaneously the furnace conditions to different products. It is impossible to modify the cycle duration because a change in the cycle would simultaneously affect all the carriages present in the furnace, which are hypothetically different. The solutions usually proposed for the regulation of heating furnaces for glass sheets are generally provided for furnaces where the successive glass sheets are identical, and thus the rates can be modified, for example, by going from one series to another, or the projected holding temperature or the power provided to each cell at the moment a new series arrives in the corresponding cell can be modified. It is even possible, as described for example in European patent No. 0047 682 for a different type of furnace, to anticipate these changes to decrease the furnace reaction time.
It is different if it is desired to produce glazings of different types without being concerned about the order in which they follow each other. Then the regulation processes proposed above have reaction times that are too long to accept significant and random variations in the weight of the glass sheets that follow each other in the furnace. In particular, with furnaces whose cells have walls that consist partially or totally of refractory bricks, the temperature cannot be rapidly adapted and, if a light windshield follows a heavier one, the second will systematically be too hot and vice versa in the opposite case.